The present invention relates to control systems for wood fiber processing machinery, and in particular to automatic controls for drum-based debarking machines that incorporate sensors and speed control mechanisms.
Debarking systems that incorporate rotating drums are known in the art. An example of such a system is taught by U.S. Pat. No. RE37,460 to Price et al., which is incorporated herein by reference. Such systems feature a large horizontal drum into which logs are inserted for debarking. The drum is fitted so as to rotate about its horizontal axis. As the drum rotates, the logs inserted within the drum rub against each other, thereby removing bark from the logs as they contact each other. The removal of bark is an essential step in the process of reducing logs to chips, which may ultimately be used in the manufacture of paper and other wood fiber products. Drum debarking may also be performed with respect to logs that are to be used for lumber.
An elevated, curved hopper is generally positioned at one end of the debarking drum, and the groups of logs to be debarked are fed into the drum using a chain-type conveyor. An auxiliary feed roller may be positioned between the chain conveyor and the drum to aid in the manipulation of longer logs through the rotating drum. A discharge conveyor is positioned on the outlet end of the rotating drum to receive debarked logs. In applications such as the creation of chips for the manufacture of paper, the material may then be feed to a chip mill conveyor for further processing of the raw wood fibers.
Conventional drum debarkers operate using simple manual controls. Before logs are to be fed into the debarker, the rotating drum and the chain conveyor are placed in the “on” position by the operator using a manual switch. In such systems, the conveyors and debarker drum are constantly in motion during operation. The speed of the conveyors, and the rate of rotation for the drum, is generally not variable. The conveyors and drum are not turned off until all of the logs and debris have moved through the system.
Simple manual operation of the debarking system has a number of disadvantages. The optimal rate of rotation for the debarking drum is determined, in part, by the number of logs within the drum at any given time. If, for example, the rate of rotation is too great for the number of logs present, then usable wood fiber material will be stripped from the logs after all bark is removed. The wood fiber lost in this manner cannot feasibly be separated from the removed bark, and thus is discarded as waste. Likewise, if the rate of rotation is too slow, then logs will be moved from the debarker without complete debarking having taken place. Since incomplete debarking is unacceptable, current practice is to simply run the debarking drum at a speed that will ensure debarking for any expected number of logs within the debarking drum at any given time. The result is wasted wood fiber material that is removed from the logs when the number of logs in the debarking drum would favor a lower speed.
The length of time that the logs remain in the debarking drum is also an important variable, which in a manual system is determined by the operator through visual inspection. If the operator leaves the logs in the drum for too long then material is wasted, but if the operator removes the logs too soon then they will have bark remaining and must be run through the debarking system a second time. Logs of varying quality and condition will require variances in the optimal debarking time. Wood variety and the season in which the debarking is performed are especially important factors in determining the optimal debarking time. Since logs of varying quality and condition will require different optimal debarking times, effective manual operation of a debarker requires considerable operator experience. Even with an experienced operator, however, the calculation of an optimal debarking time relies to some extent on guesswork. Training of a new operator requires a considerable amount of time since the new operator must obtain an intuitive feel for the nature of the logs in various conditions and in various seasons in order to operate a debarking system at acceptable efficiency.
Another disadvantage of the standard manual mode of operation for a debarking system is excessive wear on equipment. The operation of conveyors and debarking drums at full speed with no wood fiber present in the system causes friction and excessive wear of the machine components. These components are designed to operate best when material is present, but in a practical setting it is impossible to maintain an even and steady flow of material at all times during operation. An attempt to remedy this problem by constantly turning conveyors and the rotating drum off and on would also cause excessive wear of the machine components, since start-up and shutdown also causes considerable wear on the machinery. Furthermore, it would be exceedingly difficult for a human operator to constantly monitor the various components of a debarking system simultaneously and switch them on and off in an optimal manner as material moves through the system. Such a task would likely require multiple human operators.
The related art includes various attempts to develop automated control systems in the wood products industry. For example, U.S. Pat. No. 5,020,579 to Strong teaches an automatic feed control mechanism for a wood chipping machine. An infeed control circuit automatically adjusts infeed material capacity based on a load reading taken on the infeed conveyor. The control system automatically lifts a roller in the machine in order to clear jams, which are indicated by an infeed conveyor load reading that passes a certain pre-set value.
Another such device is taught by U.S. Pat. No. 6,539,993 to Starr. The system separates single logs, and then reads the diameter and volume of the logs in order to optimize debarking. A ring-style debarker is utilized. An “image” of each log is then taken, which allows an optimization of the log cutting length to be determined. Each log is then cut to length and sorted into bins of similar-length logs.
U.S. Pat. No. 6,546,979 to Jonkka teaches an automated method for controlling a drum-type debarker. This system utilizes information about both the weight of logs in the debarking drum and the rotational torque of the drum. This information is used to compute information concerning the average log density and top level of the log bunch tumbling within the drum. Alternatively, the drum weight information may be combined with optical sensing of drum filling degree in order to calculate average log density. Based on the information acquired in this manner, the system varies the speed of the drum rotation in an attempt to optimize the debarking operation. The infeed rate and discharge rate may also be varied to achieve the desired parameters. Jonkka teaches that reliance on the filling degree of the drum alone cannot produce satisfactory results in computing a proper debarking time.
The Jonkaa method offers advantages over manual control systems, but also suffers from important disadvantages. The calculations involved in this control system require precise measurement of the weight of material in the debarking drum as well as torque information related to the rotation of the debarking drum. These measurements require sensitive instruments, such as strain-gauge sensors and shaft transducers, the installation of which would involve substantial re-working of any existing debarking drum equipment already constructed. They would also substantially increase the cost of producing a new debarking drum. These limitations of the related art and others are overcome by the present invention as described below.